What Are They Like?

It has been almost 20 years since the first extrasolar planets (worlds around other stars) were discovered.  Astronomers suspected they existed, but since tiny worlds can get lost in the glare of their stars’ light, we had to wait until we had the right kinds of instruments to observe distant worlds circling other stars. Today, the number of confirmed exoplanets lies at just under 500, and that number changes almost daily.

The latest news in planet-hunting circles is the discovery of two planets orbiting a star called Kepler-9.  The planets showed up in data taken by the Kepler mission, which is tasked to search for extrasolar planets in an area of the sky in the direction of the constellation Cygnus the Swan (which is high in the northern hemisphere sky these evenings).

Now, there have been lots of extrasolar planets discovered, but this is the first time two have been confirmed orbiting the same star. The planets, named Kepler-9b and Kepler-9c, have masses that are almost that of Saturn in our own solar system. Kepler-9b is the larger of the two and it orbits the star once every 19 days. The smaller Kepler-9c lies farther away and takes 38 days to make a trip around the star.

The Kepler spacecraft observed this system to tack down the precise length of each planet’s orbit around the parent star. This isn’t the only such system that the spacecraft has data for, but it’s the first to be confirmed — a big milestone for the team and the spacecraft.

The planets were discovered as the Kepler spacecraft’s camera measured tiny decreases in the star’s brightness–decreases that occur when a planet moves between us and the star. And yes, even something so small as a planet can have an effect on the star’s brightness, and Kepler is sensitive enough to detect those little dips in the star’s light.  In addition, the distance between each planet and the star can be calculated by measuring the time between successive dips as the planet orbits the star. Small changes in the regularity of these dips can help astronomers determine the masses of planets and detect other non-transiting planets in the system. In fact, the data seem to indicate that there may be at least one other planet in the system–a world about 1.5 times the Earth’s radius that follows a scorchingly short 1.6-day orbit around the star. Kepler scientists are still taking data to make sure that this is another planet and not an anomaly in the data.

The artist’s concept of the two confirmed planets show that these are gas giant-like worlds, so they’re not likely to be places where life is going to exist — at least life as we know it. And, if there IS another planet that’s closer to Earth-size? Well, it’s not likely to be a very hospitable place either. It’s likely a scorchingly hot place with little opportunity for life to take hold.

Kepler’s discovery is the second in a pair of exoplanet finds reported this week. The European Southern Observatory announced earlier that it has found a star that may have at least five planets orbiting around it. Astronomers used the HARPS spectrograph attached to the ESO’s 3.6-meter telescope in Chile to search for the existence of planets in the system. Unlike the Kepler mission, which checks for dips in light intensity of stars with planets in orbit around them, the HARPS insrtrument looks for stellar motions that indicate the existence of planets. The amount of motion tells them the mass of the planets that may exist, and helps them deduce the orbits of those distant worlds. The measurements correspond to planets with masses between 13 and 25 Earth masses. They appear to orbit  the star with periods ranging from about 6 to 600 days. These planets lie between 0.06 and 1.4 times the Earth–Sun distance from their central star. This is actually pretty well-populated system with five massive planets located within 1.4 AU.  By comparison, the Sun has only four planets — and small ones at that — in roughly the same space.

The science of planetary detection is a major growth area in astronomy. With Kepler’s ongoing mission and the ground-based work being done at places such as ESO, I think it’s only a matter of time before the community of worlds that we can detect will be well over a thousand. Now… if we could only find one with life on it!